This invention relates to diffractive devices which generate diffraction images when illuminated by a light source. It relates particularly to diffractive devices or xe2x80x9coptically variable devicesxe2x80x9d (that is, devices which appear different when observed from different angles or under different illumination conditions) which incorporate in their surface relief structures a new type of diffractive element, hereinafter referred to as xe2x80x9cinterstitial elementsxe2x80x9d.
Throughout this specification including the claims, the terms xe2x80x9cdiffractionxe2x80x9d, diffractivexe2x80x9d and xe2x80x9cdiffractedxe2x80x9d have the meaning that includes the process whereby electromagnetic radiation in the form of light waves is scattered either coherently, incoherently, diffusely, reflectively and/or specularly from surface relief elements of the device in the form of ridges, grooves, troughs, indentations and/or protrusions of spatial and depth and/or height dimensions of extent greater than 0.05 microns, and the wavelength range of the incident and diffracted light is any single or multiple combination of wavelengths in the range of from 0.1 microns to 15 microns.
There are numerous different uses for diffractive devices of the type to which this invention relates. One common use is as a security element on banknotes, cheques, credit cards, share certificates, computer software and other valuable documents and objects. Diffractive devices are typically created by embossing a diffractive surface relief structure into metallised foil which is then adhered to the document or object, in such a way that the device cannot be removed without destroying it. In some cases, the diffractive structure can be embossed directly into the document or object. Optically variable structures cannot be photocopied, because a photocopy does not possess the optically variable characteristics of the original.
One common type of optically variable device is a hologram. Coherent laser radiation is split into two beams, one of which is directed at an object. Light reflected from the object is combined with the other beam to create an interference pattern, which is then embossed into a surface. When the surface is subsequently illuminated, an image of the object is observed. Holograms have been used on Visa(trademark) and MasterCard(trademark) credit cards since 1984.
Holograms are best observed under a single point source of light. However, credit cards are normally used in diffuse lighting conditions, such as in general daylight or under multiple fluorescent lights. As a consequence, the image observed is typically blurred and indistinct. A poor quality counterfeit hologram could therefore conceivably be passed off as an original.
Another type of optically variable device is a diffractive device composed of numerous straight-line grating elements. Straight-line gratings (and concentric circular gratings) can be produced by interference between two coherent laser beams. Parallel lines are written concurrently by the interference pattern. Kinegram(trademark) devices provide examples of optically variable devices using such technology. Relevant patent publications include European Patents 105099, 330738 and 375833. Kinegram(trademark) devices were used on the Saudi Arabian passport in 1987 and on the Austrian 5000 Schilling banknote in 1990.
An advantage of straight-line grating devices is that they provide much brighter colours than holograms, and they have considerably clearer visual effects. However, because of the diffractive properties of straight line gratings, each effect is observable only from a very narrow range of viewing angles. Moreover, standard straight line gratings are useful only for producing line-art graphics and not for portraiture which requires more flexibility in terms of grey-scale and colour effect selection.
Another type of diffractive device is the Catpix(trademark) device, used on the Australian ten dollar plastic banknote issued in 1988. The Catpix(trademark) diffractive structure consists essentially of continuous undulating lines which extend throughout the device or a substantial portion of it. The surface area is divided into notional pixels, and at each pixel the undulating lines come together to form an optical catastrophe region, which is observed by the viewer as a point of light with a fixed brightness value which depends on the spatial frequency of the lines at the catastrophe region. The points of light combine to form a diffraction image which is seen by the observer. In the case of the 1988 ten dollar note, the image was a portrait of Captain Cook.
Catpix(trademark) diffractive devices, details of which were published in European Patent Publication EP 044 9893 B1 (the contents of which are hereby incorporated herein by reference) are made using computer-controlled electron beam lithography. Each line or groove in the diffraction pattern is written individually by an electron beam. This enables precise control over the positioning and shape of each diffractive element.
Pixelgram(trademark) diffractive devices are based on an actual division of a surface relief structure into separate squares or pixels. Each pixel forms a separate diffraction grating, which may consist of straight lines or curved lines, and is responsible for generating a single point of light with a particular grey-scale and/or colour value in the diffraction image. Details of Pixelgram(trademark) technology have been published in European Patent Publication 049 0923 B1 (the contents of which are hereby incorporated herein by reference). As is the case for Catpix(trademark) devices, Pixelgram(trademark) devices are constructed using computer-controlled electron beam lithography.
An advantage of Pixelgram(trademark) devices over Catpix(trademark) devices is that the whole of each pixel area on the Pixelgram(trademark) device contributes to the light point generated by that pixel, whereas only the catastrophe region of a Catpix(trademark) notional pixel contributes to the corresponding light point. Another advantage is that pixels may have radically different orientations from those of surrounding pixels, so that more than one diffraction image can be generated simultaneously. However, there are inevitable discontinuities between adjacent pixel gratings, and these result in extraneous diffuse scattering effects. Diffuse scattering from the edges of pixels has the effect of reducing the brightness of the observed optical effects, particularly for very small pixel sizes. Moreover, because of the clear separation between pixels, it is potentially easier for a person to decode the optical effects mechanisms of Pixelgram(trademark) structures than Catpix(trademark) structures, using microscopic examination of the structures.
Exelgram(trademark) diffractive devices consist of diffractive elements arranged in tracks. Diffractive elements, which may be grooves, ridges, or polygonally shaped indentations or protrusions, are arranged in substantially continuous configuration along the tracks. Details of Exelgram(trademark) technology have been published in U.S. Pat. No. 5,825,547 (the contents of which are hereby incorporated herein by reference). As is the case for Catpix(trademark) and Pixelgram(trademark) devices, Exelgram(trademark) devices are typically created using electron beam lithography.
Exelgram(trademark) devices, which were first used on Australian opal stamps in 1995 and American Express Travellers"" cheques in 1997, overcome part of the extraneous diffuse scattering effect problem apparent in Pixelgram(trademark) devices; however, there are still extraneous diffuse scattering effects caused at the interface between tracks.
According to the present invention, there is provided a diffractive device having a surface relief structure which, when illuminated by a light source, generates one or more diffraction images which are observable from particular ranges of viewing angles around the device, including:
background diffractive structural elements; and
interstitial diffractive structural elements;
wherein the interstitial elements are interspersed between the background elements such that the diffractive action of the background elements is modulated by the interstitial elements, with differing interstitial element configuration in differing parts of the surface relief structure producing differing diffraction effects in corresponding parts of the diffraction images.
In one arrangement, at least some of the background elements may consist of a multiplicity of continuously connected individual ridge or groove segments, with ridge or groove segments in adjacent background elements being arranged in an approximately parallel configuration. In this arrangement at least some of the interstitial elements consist of individual or bifurcated ridge or groove segments interspersed between the background elements, with interstitial element ridge or groove segments being approximately parallel to ridge or groove segments in adjacent background elements.
In an alternative arrangement, at least some of the background elements may be approximately parallel, each consisting of a plurality of discontinuous individual ridge or groove segments. In this arrangement, at least some of the interstitial elements are approximately parallel to each other, each consisting of one or more ridge or groove segments and each being located in a discontinuity in a background element. Some of the interstitial elements may be oriented generally at right-angles to the general orientation of the background elements, in order to provide one or more images which are observable from positions at right-angles to the positions from which images produced by the background elements can be observed.
The interstitial elements may be separate structural elements, or they may be joined to other structural elements. It is preferred that at least some of the interstitial elements are connected smoothly to adjacent interstitial elements and/or background elements in one or more of the following ways:
(a) a single interstitial element bifurcates smoothly into two interstitial elements;
(b) two interstitial elements join smoothly into a single interstitial element;
(c) an interstitial element joins smoothly into a background element;
(d) an interstitial element of a particular depth and width transitions smoothly into an interstitial element of a different depth and width;
(e) an interstitial element of a particular shape and/or curvature transitions smoothly into an interstitial element of a different shape and/or curvature;
(f) an interstitial element with a particular angular orientation relative to the background elements joins smoothly to an interstitial element having a different angular orientation.
Similarly, some of the background elements may be connected smoothly to adjacent background elements and/or interstitial elements in one or more of the following ways:
(a) a single background element bifurcates smoothly into two background elements;
(b) two background elements join smoothly into a single background element;
(c) a background element joins smoothly into an interstitial element;
(d) a background element of a particular depth and width transitions smoothly into a background element of a different depth and width;
(e) a background element of a particular shape and/or curvature transitions smoothly into a background element of a different shape and/or curvature;
(f) a background element with a particular angular orientation relative to other background elements joins smoothly to a background element having a different angular orientation.
The background diffractive structural elements and the interstitial diffractive structural elements may have any suitable shapes and configurations. It is preferred that each of the background elements and the interstitial elements has a shape which is one or more of:
(a) a straight, curved or undulating groove;
(b) a straight, curved or undulating ridge;
(c) an array of dot-shaped indentations or protrusions; or
(d) a polygonally shaped indentation or protrusion.
As indicated above, the interstitial element configuration is responsible for the particular optical effects produced by the surface relief structure. It is preferred that the diffraction effects observed in a particular part of the image are determined by the interstitial element configuration in a corresponding part of the surface relief structure, and the interstitial element configuration features include:
(a) lengths of interstitial elements;
(b) widths of interstitial elements;
(c) depths and/or heights of interstitial elements;
(d) local spatial frequency of interstitial elements;
(e) degree of curvature of interstitial elements;
(f) shape of interstitial elements; and
(g) shapes of joins between adjacent interstitial elements.
In order to produce particular optical effects, the interstitial elements in a particular region may all have the same shape, orientation and spatial frequency. However, in order to achieve other types of effects, the interstitial elements may vary continuously in terms of orientation, curvature, thickness and/or shape, the variations being a means by which image information is encoded into the surface relief structure.
In a typical arrangement of the present invention, the background to elements generate a xe2x80x9ccarrier wavexe2x80x9d and the interstitial elements modulate the carrier wave according to the information which is encoded into them.
The interstitial elements may be oriented in any suitable manner relative to the background elements. It is preferred that at least some of the interstitial elements are oriented generally parallel to the background elements. It is also possible that at least some of the interstitial elements are arranged in a comb-like configuration, with the teeth of the comb being oriented at right angles or at an angle oblique to the general orientation of the background elements. Alternatively or additionally to the comb-type configurations, at least some of the interstitial elements may be arranged in groups oriented at right angles or obliquely to the general orientation of the background elements, such that a cross-section through the group has a periodic or sinusoidal shape of many repeating periods or oscillations. In general, a group of parallel diffractive elements produces a diffraction effect which is observable from a direction perpendicular to the orientation of the diffractive elements. Thus a surface relief structure which includes both interstitial elements arranged parallel to the background elements and interstitial elements arranged at right angles to the background elements will produce at least two different images, observable from directions which are at rights angles to each other. Interstitial elements arranged in other oblique orientations will produce diffraction effects observable from correspondingly different directions.
It is preferred that at least some of the interstitial element configurations are designed to create grey-scale or variable image intensity information in the image, and one or more of the following configuration features give rise to the grey-scale or variable image intensity information:
(a) lengths of interstitial elements;
(b) degree of curvature of interstitial elements;
(c) widths of interstitial elements and shapes of joins between adjacent elements;
(d) local slope or angle of interstitial elements.
It is further preferred that at least some interstitial element configurations are designed to create colour information in the image. Colour information is typically dependent on the spatial frequencies of the interstitial elements (the number of substantially parallel diffractive elements per unit of distance).
The background elements may be of any suitable shape and configuration. It is preferred that the background elements are in one or more of the following configurations:
(a) straight, equally spaced background elements;
(b) straight, variably spaced background elements;
(c) undulating, equally spaced background elements;
(d) undulating, variably spaced background elements;
(e) equally spaced closed or open elliptically shaped background elements;
(f) variably spaced closed or open elliptically shaped background elements;
(g) zig-zag shaped background elements of variable zig or zag angle.
In some embodiments of the invention, the surface relief structure may generate two or more diffraction images which are observable from different ranges of viewing angles. In such embodiments, some regions of the surface relief structure contribute to one of the images, and other regions contribute to another of the images.
The limit of resolution of a human eye when viewing an object at close quarters is typically about 0.25 mm. It is desirable that at least some component parts of images generated by the surface relief structure be smaller than this so that the images appear smooth. There are also security advantages to having components such as microwriting and micrographics which are too small to be observed except with a magnifying glass or microscope. It is therefore preferred that at least some of the interstitial elements have lengths of less than 0.25 mm. It is especially preferred that some elements have dimensions in the range of 4 to 30 micron. On the other hand, it is preferred that at least some of the background elements have elements extend throughout the length or width of the surface relief structure.
In addition to the features discussed above, there are numerous items which can be incorporated into the surface relief structure. In some arrangements, the surface relief structure may include between background elements one or more of the following:
(a) small scale text or graphics indented into or protruding from the surface relief structure;
(b) interstitial elements consisting of parallelograms of varying angular orientations indented into the surface relief structure;
(c) diffusely reflecting randomly distributed interstitial elements;
(d) diffusely reflecting trapezoidal interstitial elements.
It will be appreciated that the present invention has several advantages over the prior art Hologram, Kinegram(trademark), Catpix(trademark), Pixelgram(trademark) and Exelgram(trademark) structures. These include:
(a) The background diffractive structural elements provide a xe2x80x9ccarrier wavexe2x80x9d which has continuous connectivity throughout the structure, thereby minimising the diffuse scattering edge effects when compared to similar effects that arise from the edges of the pixels in Pixelgram(trademark) and Kinegram(trademark) structures and along the edges of tracks in Exelgram(trademark) structures. Diffuse scattering effects from the edges of pixellated devices of very small pixel size have the effect of reducing the brightness of the observed optical effects.
(b) The interstitial structural elements are located within the area of the background xe2x80x9ccarrier wavexe2x80x9d structural elements, so the optical characteristics of each region are the result of a complex interaction between the diffractive properties of both the interstitial elements and the background structural elements. This embedding of interstitial elements within a continuous background structural element pattern makes it much more difficult to de-code the optical effects mechanisms from a microscopic examination of the surface relief structure. The surface relief structure is therefore more secure with respect to attempted re-origination by reverse engineering of the optical microstructure.
(c) Since the background structural element pattern is continuous in both directions, the image resolution of any image encoded into the microstructure is determined by the relative sizes of the interstitial element regions. The size of each interstitial region is determined by the number of interstitial elements and the lengths of these elements, and these are continuously variable. Thus the microstructure has the unique property that the encoded image resolution is continuously variable throughout the surface relief structure.
(d) Interstitial xe2x80x9cdopingxe2x80x9d of a continuous background structural element pattern by a multiplicity of structural element xe2x80x9cfilamentsxe2x80x9d of variable length, width, curvature and depth allows for the incorporation of a much greater range of specialised optical security effects at a much smaller scale when compared to diffractive microstructures based on a pixellated format. The fundamental optical information unit in the present invention is a structural element, whereas in pixellated devices the fundamental optical information unit is a pixel consisting of a relatively large number of structural elements. Hence interstitial element structures offer the possibility of designing xe2x80x9coptically variable devicexe2x80x9d images with much higher resolution optical effects.
(e) Because interstitial element microstructures require individual fabrication of each groove element, the fabrication of the microstructure requires advanced electron beam lithography or some other advanced method. Holographic origination is not possible, so the availability of the technology is much more restricted, and security printing industry requirements for controlled access to the technology are much more achievable.
Although the invention has been described above primarily with reference to application in providing visually observable images, the invention also has application to machine-readable data and security applications. Machine-readable digital information may be encoded into the positioning, length, orientation and/or other physical characteristics of interstitial elements, such that the information may be read by passing a laser over the interstitial elements and analysing and decoding the reflected light.